The present invention relates to a self-discharging centrifugal drum for clarifying and separating suspensions that has an axially displaceable piston valve that opens and closes expulsion openings in the circumference of the drum jacket to allow the removal of centrifuged solids, in which the top of the piston valve is demarcated by the solids space and its bottom by a closure compartment into which closure fluid can be pumped, in which the closure fluid is supplied through a channel that is connected to the closure compartment and part of which is an intermediate compartment for closure fluid, and in which there is a valve that is activated by control fluid to evacuate the closure compartment in such a way that, when the valve is activated to withdraw closure fluid from the closure compartment, the closure pressure in the closure compartment is reduced and part of the closure fluid remains in the intermediate compartment.
A centrifugal drum of this type is known for example from German Offenlegungsschrift 3 021 638. In this device an annular valve in the closure compartment axially displaces an annular elastic element that seals the closure compartment off from the outlet to open an outlet gap for closure fluid at the inlet into the closure compartment while the elastic element simultaneously blocks off the connection between the closure compartment and the intermediate compartment.
Uniform delivery of the drum packing and especially of the solids centrifuged into the periphery of the solids space requires both more rapid opening, with a large opening gap between the piston valve and the sealing site in the drum cover, and more rapid closure of the piston valve, especially when discharge is partial.
To obtain a uniformly large opening gap in the drum, which will prevent caked-up solids from getting clogged up in it, the piston valve should travel through a uniform stroke during both total and partial discharge. One practical way to obtain such uniformity is to ensure total evacuation of the closure compartment, which must then be refilled with closure fluid at a rate that depends on the desired level to which the drum is to be discharged.
Although known centrifugal drums, which by the way utilize a very expensive valve, can be opened and closed rapidly, it is extremely difficult to control the opening time. Since the amount of closure fluid in the closure compartment under the piston valve is always constant, it will force the piston valve back into the closure position as soon as the pressure of the closure fluid under the piston valve again becomes somewhat higher than that of the centrifuged fluid in the solids space, which acts on the top of the piston valve. It is therefore also hardly possible with this drum to obtain either complete total discharge or different levels of partial discharge of the drum packing. Furthermore, the constant deformation of the elastic closure element will subject it to extreme wear.